A single entry point in front of your services that handles authentication, routing, rate limits, and logging, so each service does not implement them separately.
It gives you one place to enforce access and one place to see traffic — and it lets you restructure services behind it without breaking the clients already integrated.
We put a gateway in front of anything with several consumers or public access, keep authentication and quotas there, and keep business logic out of it — a gateway that makes decisions becomes the bottleneck.